1. Field of the Invention
The present invention generally relates to a rechargeable battery arrangement including a rechargeable battery, a rechargeable battery pack, etc. and more particularly, to a lithium ion secondary battery (lithium battery) and its rechargeable battery pack, which each include an internal circuit capable of externally outputting a residual capacity of the battery corresponding to a state of use of the battery.
2. Description of the Prior Art
Conventionally, when a lithium battery or the like has been discharged, the storage battery is initially charged at constant current. Then, when the storage battery has been considerably charged to a substantially full charging state, its charging is changed over to that at constant voltage. During charging of the storage battery at constant voltage, when charging current has reached not more than a predetermined value or charging voltage has reached not less than a predetermined value, a charging switch is turned off by judging that the storage battery has been charged fully, thereby resulting in termination of the charging. In order to prevent overcharge, a controller for controlling charging or a portion of its circuit is incorporated into or provided integrally with a lithium ion battery or its rechargeable battery pack.
The rechargeable battery or the rechargeable battery pack of this kind is incorporated into an electronic device such as a portable computer, a handheld electronic device or the like. When voltage of the rechargeable battery has dropped to not more than a predetermined value, the rechargeable battery is charged by a charging circuit in the electronic device. When charging of the rechargeable battery by charging current has been completed, the charging is terminated. Then, when the electronic device is driven by the battery, the battery performs discharge so as to supply electric power to the electronic device. To this end, a controller for controlling charging is provided with a relay or a relay device for effecting bidirectional flow of electric current between a positive electrode of the rechargeable battery and a charging terminal so as to switch direction of electric current. Meanwhile, a diode is inserted in series in a direction of each of charging and discharge so as to have a diode changeover circuit which selects electric current in one direction and blocks electric current in the opposite direction.
When the electronic device having the rechargeable battery of this kind is connected to an AC power source and is not being operated or is being operated, the rechargeable battery is charged. When the electronic device is operated by disconnecting the electronic device from the AC power source, the electronic device is operated by electric power from the rechargeable battery.
Recently, batteries conforming to smart battery standards have been developed and used as batteries to be incorporated into electronic devices of this kind. In the smart battery standards, a processor (MPU) in the electronic device and a control circuit which is provided as an internal circuit of the rechargeable battery and includes a further processor (MPU) are connected to each other by an SM (system management) bus such that the further processor of the rechargeable battery can transmit data on states of the rechargeable battery to the processor of the electronic device. One of such data on states of the rechargeable battery includes data on a residual capacity of the rechargeable battery. The data on the residual capacity of the rechargeable battery is usually used for judging whether or not data processing scheduled in the electronic device can be performed without malfunction by the present residual capacity of the rechargeable battery.
In the conventional battery conforming to the smart battery standards, in case the data on a residual capacity of the rechargeable battery is transmitted to the electronic device, discharge current is detected each time the electronic device has been used so as to obtain a quantity of consumed electricity, i.e., a quantity of discharged electricity from current consumed between a point of full charging and the present such that the residual capacity of the rechargeable battery is obtained by subtracting the obtained quantity of consumed electricity from a total service capacity up to a preset final discharge voltage, for example, a fixed total service capacity Q.sub.0 having 3.0 V as the final discharge voltage.
However, if discharge conditions such as discharge current, temperature, etc. vary, the total service capacity up to termination of discharge also varies, thereby resulting in variations of residual capacity of the discharge battery. In case such a phenomenon is not taken into consideration, the residual capacity of the rechargeable battery should be calculated so as to include allowance for variations of the discharge conditions. Therefore, such a problem arises that since the calculated residual capacity of the rechargeable battery becomes inaccurate and the residual capacity of the rechargeable battery is outputted in a state where the rechargeable battery has not been used sufficiently, charging of the rechargeable battery is repeated many times and thus, operating efficiency of the rechargeable battery deteriorates on the contrary.